To study the possible cellular origin of recently recognized indolent termi
nal deoxynucleotidyl transferase (TdT)-positive T-lymphoblastic proliferati
ons of the tonsils and oropharynx, we studied normal human tonsils for the
presence of TdT-positive cells. TdT-positive cells were readily demonstrate
d in the tonsils from 15 children and adults by immunohistochemical stainin
g TdT-positive cells were distributed in discrete foci at the periphery of
lobules of lymphoid tissue and adjacent to fibrous septa and had the morpho
logic features of small to medium-sized lymphocytes. Double-antibody staini
ng indicated the TdT-positive cells had the phenorype ofuncornmitted early
I?imphoidprecursors (CD3-, CD79a-, CD10-). Foci of TdT-positive cells were
not identified in 6 reactive lymph nodes studied as controls. These studies
indicate that tonsils, like bone marrow and thymus, are sires of lymphopoi
esis. The presence of TdT-gositive precursor cells in human tonsils may be
a factor in the pathogenesis of recently described indolent T-lymphoblastic
proliferations involving the tonsils and oropharynx. The presence of TdT-p
ositive cells in human tonsils should not be misinterpreted as evidence of
lymphoblastic lymphoma or leukemia.